TRANSMISSION ELECTRON-MICROSCOPY MICROSTRUCTURE CHARACTERIZATION OF YBCO SRRUO3/YBCO JOSEPHSON-JUNCTIONS/

The microstructure of YBCO/SrRuO3/YBCO junctions in the edge geometry have been studied by high resolution electron microscopy. Various defe cts in the YBCO and SrRuO3 films are observed and analyzed. The most s triking defects in the c-axis-oriented YBCO film are a-axis particles, which cause bending and interruption of lattice planes as well as hig h steps and cracks in the subsequent layers. The SrRuO3 layer in the r amp-edge junction region consists of small domains with slightly diffe rent orientations. This leads to the formation of steps on its surface and large variation in SrRuO3 thickness. Mechanisms for forming this defective layer are suggested based on the growth mode and the crystal structure of SrRuO3. We find evidence for strain fields at the SrRuO3 /YBCO interface. The oxygen deficiency in YBCO caused by this interfac e strain may to a large part be responsible for the excess resistivity of this junction. In addition to the defective SrRuO3 layer, the occu rrence of various defects in the junction area, including Y2O3 and a-a xis YBCO particles, seem to be the cause for the wide scatter in the j unction electrical characteristics. The formation of steps and large t hickness variation in the barrier layer may be the reason for the rath er large critical thickness of the SrRuO, superconductor-normal-superc onductor (SNS) junction. In addition to the junction interfaces, the m icrostructure of SrRuO3/LaA1O(3) and SrRuO3/SrTiO3 interfaces have als o been studied.